No 4 (2019)

Structure and evolution of the Eastern part of the Southwest Indian Ridge
Kokhan A.V., Dubinin E.P., Sushchevskaya N.M.

The structure and evolution of eastern part of the Southwest Indian ridge is discussed. Based on geological-geophysical data and cartographic materials, analysis of spatial and temporal changes of ridge morphostructure was performed. Analysis of the data made it possible to recognize the stages of ridge evolution and delineate its sections differing in tectonics, structure, morphostructural segmentation of the rift zone and the ridge flanks. It is shown that crust in the axial zone of the ridge formed under repeatedly changing spreading kinematics, degree of hotspot activity, and underlying mantle along-axis temperatures. The temporal variations in these characteristics along the rift zone are established, with consideration of their influence on its segmentation and tectonic structural features.

Геотектоника. 2019;(4):3-24
Tectonics of the Southern Ocean passive margins in the Africa – East Antarctica region
Melankholina E.N., Sushchevskaya N.M.

Based on geological and geophysical data for the conjugate margins of Africa – East Antarctica, the peculiarities of preparation of the breakup central Gondwana supercontinent are discussed. When using the historical approach, a significant inheritance of the Middle-Upper Jurassic tectono-magmatic development from the preceding time is shown. The first location of tectono-magmatic activity in zones of weakness on the proximal margin, its subsequent migration to distal margins and further oceanic opening is established. The geochemical features of magmas of the region and their sources are under discussion. Evidence for the decisive influence of the Karoo-Mod plume on the development of magmatism is presented. A significant feature of the plume manifestation is considered: the presence of high-magnesian ferruginous picrites , formed by melting of a pyroxenite source with specific composition, coinciding with the central part of the plume and corresponding to the earliest eruptions. We determined the source of magmatism at the initial stage could have been the substance of a rising plume, and magmas reached the surface through existing fractures without interacting with the lithosphere. In the course of evolution, the admixture of pyroxenites in the source decreased and the melts acquired the features of the melting lithospheric mantle, which was reflected in the isotopic characteristics of the melts with a predominance of the enriched EM II component. The structure and magmatism of the Southern Ocean and South Atlantic are compared. Also discussed the locations of the Mesozoic Karoo-Maud and Tristan plumes, as well as the zones of the subsequent breakup of Gondwana, above the margin of the African superplume, indicating a relationship between surface and deep-seated events, is discussed.

Геотектоника. 2019;(4):25-42
Reconstruction of the Vendian–Cambrian active continental margin of the Southern Urals: results of studying of detrital zircon from the Ordovician terrigenous rocks
Ryazantsev A.V., Kuznetsov N.B., Degtyarev K.E., Romanyuk T.V., Tolmacheva T.Y., Belousova E.A.

Detrital zircons of Ordovician terrigenous sequences are studied in various Southern Uralian tectonic units.The age of detrital zircons of the West Uralian and Transuralian megazones, Taganai–Beloretsk Zone, and Kraka allochthons spans from the Late Archean to the end of the Vendian– beginning of the Cambrian; Early Precambrian and Early–Middle Riphean zircons are the most abundant. Vendian–Cambrian detrital zircons are strongly dominant in the Uraltau Zone, Sakmara allochthons, and East Uralian Megazone; the zircons of other ages are absent or extremely rare. The Vendian–Cambrian detrital zircons of all Southern Urals zones probably derive from volcanic and granitic rocks of the marginal continental belt, which are part of the Uraltau Zone, Sakmara allochthons, and East Uralian Megazone. The Lu–Hf isotopic characteristics of Vendian–Cambrian detrital zircons indicate that their parental rocks formed on a heterogeneous basement that includes blocks of juvenile and ancient continental crust. According to a model of the pre-Ordovician tectonic evolution of the Southern Urals, at the end of the Late Riphean, the passive margin of the East European Platform collided with a block on a heterogeneous basement. The formation of the block terminated with the Grenville Orogeny. After collision, a volcano-plutonic belt originated in the Vendian–Cambrian at the actively evolved margin of the East European Platform.

Геотектоника. 2019;(4):43-59
Late Mesozoic East Asian magmatic province: structure, magmatic signature, formation conditions
Yarmolyuk V.V., Nikiforov A.V., Kozlovsky A.M., Kudryashova E.A.

The Late Mesozoic igneous province of East Asia in connection with global geological events is considered. The main structure-forming events and the largest magmatic productivity of the province coincided with the peak of widely manifested plume activity in the Early Cretaceous. A geodynamic model of the magmatic province formation is proposed, relating development of the province with the complex geodynamic setting for the interaction of the convergent boundary with the hot mantle field. The Pacific marginal magmatic belt was formed in front zone of convergence where accretion of terranes occurred with prevalence of supersubduction magma-forming mechanisms. In the western part of the province outside of convergence zone an intraplate volcanic areas were formed due to activity of small mantle plumes.

Геотектоника. 2019;(4):60-77
Recent mountain-building at the boundary juction of the North-Western Caucasus and intermediate Kerch-Taman Region, Russia
Trikhunkov Y.I., Bachmanov D.М., Gaydalenok О.V., Marinin А.V., Sokolov S.А.

In this paper the recent evolution of marginal segment of the North-Western Caucasus orogen in its junction to Kerch-Taman periclinal trough was studied. Geomorphological analysis included geological, tectonophysics data and digital elevation models was carried out. The North-Western Caucasus torn-folded morphostructures that had their extension in intermediate Kerch-Taman region westward to North-Western Caucasus mountain structure are both established to be developed in uniform regional compression settings. The prevailing horizontal compression direction changed from N–E in the North-Western Caucasus to the meridional one in Taman region, which was expressed in the reorientation of the axes of the folded morphostructures and geophysical anomalies from N–W to the latitudinal direction. The separation of the compressional vectors occurred in Pliocene-Quaternary period. Pre-Pliocene fault structures of N–E strike in this area were replaced with meridional Anapa-Dzhiginka and Abrau active fault zones.We compared Abrau zone with Anapa-Dzhiginka zone and established the faulting in Abrau zone became more dynamic in Pliocene-Quaternary stage, we revealed clear signs of vertical amplitude droppable-sliding kinematics of 500–600 m of normal fault deformation in Abrau zone during this stage. Abrau zone is the Greater Caucasus western boundary zone.The possible reasons for the restructuring of regional structural geometry and lateral transition activity of the faults transverse to the Northern-Western Caucasus orogen, are considered.

Геотектоника. 2019;(4):78-96

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